The present invention relates to a catalyst used when producing a synthesis gas containing carbon monoxide and, hydrogen becoming a raw material gas of GTL, DME, methanol, ammonia, hydrogen production and the like by conducting partial oxidation by adding oxygen to a light hydrocarbon such as a natural gas containing methane and hydrocarbon having 2 or more carbon atoms, and the associated gases, and a process for producing a synthesis gas.
From the fact that global environmental problems due to mass consumption of fossil fuels such as oils and coals and depletion problems of oil resources in future are recently taken up, GTL (hydrocarbon liquid fuel) and DME (dimethyl ether) that are clean fuels produced from a natural gas and the like are noted. A raw material gas that produces GTL and DME is called a synthesis gas, and contains carbon monoxide and hydrogen.
A steam reforming (SMR) that reforms a natural gas or the like with steam, a partial oxidation (PDX) using oxygen in the absence of a catalyst, or an autothermal reforming (ATR) in which oxidation reaction using oxygen burner and a steam reforming reaction are conducted in the same reactor is conventionally known as a method of producing the synthesis gas. The applicant of the present application develops new process for producing a synthesis gas employing a catalytic partial oxidation (CPO) in which apparatus constitution is simple as compared with the conventional methods and the problems of generation of soot and carbon deposition during reaction are reduced.
The CPO is a method for obtaining a synthesis gas by contacting a hydrocarbon gas separated from a natural gas or the like with an oxygen-containing gas in the presence of a catalyst to partially oxidize the hydrocarbon gas (Patent Document 1). The CPO is excellent as compared with the autothermal reforming in that because a burner is not used, a pre-reformer is not required even though components of C2 or more higher are contained. Furthermore, the CPO has the advantage that because the rate of reaction is extremely large, a reaction is completed even under high GHSV hydrocarbons condition of several ten thousands to several millions, and as a result, a size of a reactor is decreased.
For example, in the case of methane, the reaction mainly includes the following reactions.
(1)CH4 + 1/2O2 → 2H2 + COΔH298 = −36 kJ/mol(2)CH4 + 2O2 → CO2 + 2H2OΔH298 = −879 kJ/mol(3)CO + H2O → CO2 + H2ΔH298 = −42 kJ/mol(4)CH4 + H2O → CO + 3H2ΔH298 = +206 kJ/mol(5)CH4 + CO2 → 2CO + 2H2ΔH298 = +248 kJ/mol
The reactions (1) to (5) proceed in combination or sequentially, and a gas composition at an outlet is governed by the equilibrium. However, those reactions are very large exothermic reaction as the whole reaction. Of those reactions, reaction rate of the reactions (1) and (2) is extremely large, and particularly, reaction heat of complete oxidation of (2) is large. As a result, temperature is rapidly increased at an inlet of a catalyst bed. The solid line shown in FIG. 8 is temperature distribution in which a horizontal axis shows the position of from an inlet side to an outlet side of a reactor and a vertical axis shows temperature of a catalyst bed. For example, when a feed gas is supplied at a temperature of about 200 to 300° C., the feed gas is affected by the exothermic reaction, and the temperature of an inlet part of a catalyst bed is rapidly increased to, for example, about 1,200 to 1,500° C. Then, by the influence of endothermic reaction, for example, (4) and (5), in which the reaction rate is relatively small, the temperature of the catalyst bed is gradually decreased, and it then reaches a condition of thermal equilibrium at about 1,000° C.
The temperature distribution of a catalyst bed rapidly varies depending on, for example, change in composition of a feed gas and very small change in pressure, and the position at which the temperature of a catalyst bed is maximum shifts to an upstream side or a downstream side of a reaction according to those changes as shown by, for example, a broken line or an alternate long and shot dash line in FIG. 8. For example, in the case that the temperature distribution in the reactor is changed from the state shown by the solid line to the state shown by the broken line in FIG. 8, a catalyst charged at the position shown by point A on the horizontal axis is rapidly heated to about 1,200 to 1,500° C. The temperature change occurs in, for example, from less than 1 second to several seconds. Therefore, the catalyst in this region is exposed to rapid temperature change of, for example, about 250° C./second to 1,300° C./second. On the other hand, in the case that the temperature distribution shown by the solid line is changed to the temperature distribution shown by the alternate long and short dash line, the catalyst charged at the position of, for example, point B is cooled and rapidly exposed to temperature change to the same extent as the point A.
The change of temperature distribution in the catalyst bed is intermittently generated during operation of the reactor. Therefore, the catalyst charged in the vicinity of the inlet of the reactor always repeatedly undergoes the rapid heating and cooling. The catalyst charged in the reactor is formed into a Sphere, a tablet, a Cylinder, a honeycomb, a monolith, a ring, guaze, a foam or the like. As a result, the catalyst undergoes stress change by rapid expansion and contraction posed by the heating and cooling, that is, thermal impact, and as a result, is destroyed and powdered, resulting in clogging of the catalyst bed. Where the catalyst layer causes clogging, pressure drop of the reactor is increased, and there is a possibility that the operation cannot be continued. For this reason, the catalyst used in the CPO is required to have high thermal shock resistance.
Patent Document 2 describes a catalyst for use in the CPO, having improved thermal shock resistance by supporting an active metal on a carrier comprising zirconia as a main component. However, the catalyst applies temperature change of 60 to 100° C./second over a temperature range of from 800 to 1,200° C. to the application range. This temperature change is very mild condition as compared with the above-described temperature change, and is not suitable to the CPO process developed by the present applicant.
Patent Document 1 JP-A 2007-69151: paragraphs 0038 to 0042.
Patent Document 2 Japanese Patent No. 4020428: Claim 1, paragraph 0027.